The present invention relates generally to rotary machines and, more particularly, to actuated seals for rotary machines.
Rotary machines include, without limitation, steam turbines, gas turbines, and compressors. A steam turbine has a steam path that typically includes, in serial-flow relationship, a steam inlet, a turbine, and a steam outlet. A gas turbine has a gas path which typically includes, in serial-flow relationship, an air intake (or inlet), a compressor, a combustor, a turbine, and a gas outlet (or exhaust nozzle). Gas or steam leakage, either out of the gas or steam path or into the gas or steam path, from an area of higher pressure to an area of lower pressure, is generally undesirable. For example, a gas path leakage in the turbine or compressor area of a gas turbine, between the rotor of the turbine or compressor and the circumferentially surrounding turbine or compressor casing, will lower the efficiency of the gas turbine leading to increased fuel costs. Also, steam-path leakage in the turbine area of a steam turbine, between the rotor of the turbine and the circumferentially surrounding casing, will lower the efficiency of the steam turbine leading to increased fuel costs.
It is known in the art of steam turbines to position, singly or in combination, labyrinth-seal segments with or without brush seals, in a circumferential array between the rotor of the turbine and the circumferentially surrounding casing to minimize steam-path leakage. Springs hold the segments radially inward against surfaces on the casing that establish radial clearance between the seal and rotor but allow segments to move radially outward in the event of rotor contact. While labyrinth seals, singly or in combination with brush seals, have proved to be quite reliable, labyrinth-seal performance degrades over time as a result of transient events in which the stationary and rotating components interfere, rubbing the labyrinth teeth into a “mushroom” profile and opening the seal clearance.
One means of reducing the degradation due to rubbing has been to employ “positive-pressure” variable-clearance labyrinth packings, in which springs are used to hold the packing-ring segments open under the no- or low-flow conditions during which times such rubbing is most likely to occur. Ambient steam forces overcome the springs at higher load acting to close the rings to a close running position. However, it would be desirable to provide an “actively controlled” variable-clearance arrangement in which the packing-ring segments are held open against springs and steam force by internal actuators, during the conditions under which rubbing is most likely to occur. At the operating conditions under which rubbing is unlikely, actuator force could be reduced, permitting the springs and steam forces to move the segments to their close running position.
In order to actuate such ‘active’ or ‘adjustable’ seals against the steam force, high pressures within the actuators are often required. Additionally, in certain situations when the seals need to be opened quickly, high pressure must be built up inside the actuators in a very short period of time. Problems arise, however, in that excessively high pressure differentials in the actuators tend to reduce their useful life. Additionally, due to compressibility of the actuating medium, such as the case with air or other gases or liquid, the time that it takes to build the pressure inside the actuators may be longer than what is desired to actuate, and thereby protect, the seals. Additionally, in certain situations when the turbine steam pressure falls, it is desirable to depressurize the actuators accordingly to avoid excess pressure in the actuators with respect to the ambient steam pressure. Due the compressibility of the actuating medium, if this venting process takes too long, the excess pressure in actuators may reduce their useful life.